A. Field of the Invention
The invention relates generally to the transmission of data within a video signal and, more particularly, it relates to the transmission of quadrature amplitude modulated digital data within the vertical and horizontal overscan portions of an NTSC TV signal.
B. Discussion of Prior Art
The active video portion of one horizontal line of an NTSC TV signal is approximately 52 .mu.s while the active video portion in the vertical direction is approximately 12 ms. However, a typical NTSC TV receiver overscans, in both the horizontal and vertical direction, the transmitted image by approximately 6%, 3% on either side of the picture. This overscan causes the right and left edge, as well as the top and bottom, of the picture to remain hidden from view. The overscan interval amounts to approximately 1.5 .mu.s of the active period on either side of the signal line and approximately 730 .mu.s of the active period on the top and bottom.
This unseen portion of the NTSC signal provides an opportunity to transmit other information during the horizontal and vertical overscan portion of the signal. To date, designers of wide-screen TV receivers have used the horizontal overscan portion to transmit side panel information which, along with the center panel information, is converted into a wide-screen TV image.
An example of the above use of the overscan portion of the NTSC TV signal is found in (1) Inoue et al., "Encoding and Decoding in the 6-MHz NTSC-Compatible Widescreen Television System", IEEE Transactions on Circuits and Systems for Video Technology, Vol. 1, No. 1, Mar. 1991, pp. 49-58.
Other examples are found in U.S. Pat. No. 4,552,754 issued to Meise et al. and U.S. Pat. No. 4,888,641 issued to Isnardi et al. both of which are hereby incorporated by reference for their teachings on the design of television systems. In Meise et al., the side panel information for the wide-screen image is time compressed for transmission during the horizontal overscan (see col 3, lines 48 to 64). And, in Isnardi et al., an auxiliary channel is used for transmitting auxiliary video information to be used with a widescreen HDTV system (see cols 4-5 with reference to FIG. 1 and see FIG. 3a).
However, the overscan portion of the NTSC TV signal need not be limited to transmission of side panel information for wide-screen receivers. This transmission time period can be used to transmit many different types of information, whether digital or analog, for many different purposes.